1. Field of the Invention
The present invention relates to carbon fiber reinforced thermotropic liquid crystalline polymers, and more particularly, to the production of prepregs and composites from such reinforced thermotropic liquid crystalline polymers.
2. Description of the Prior Art
Structural composites made from continuous carbon fibers and thermoset resins are well-known in the prior art. Thermoset resins are low in viscosity and thus are readily used to impregnate fiber tows to form a useful intermediate product form which is called a prepreg. Such a prepreg has tack so that plies can readily adhere to one another, and drape so that parts with various shapes can be prepared. After being cured, such thermosets provide good elevated temperature performance, chemical and solvent resistance, and high dimensional stability. However, thermoset resins also have several disadvantages which include: long cure times, a need for refrigeration since they advance significantly at room temperature, a very brittle nature after curing, and an inability to use scrap since the cured thermosets are infusible.
In contrast, thermoplastic resins have several advantages over thermoset resins when used with carbon fibers. They exhibit excellent impact strength, can be formed quickly without the need for a curing process, need no refrigeration, and the scrap can be reused. In addition, thermotropic liquid crystalline polymers, a particular type of thermoplastic resin, has good solvent resistance and creep resistance, as well as good elevated temperature performance.
Prior to the present invention, obtaining such a structural composite has been difficult due to the relatively high viscosity of such resins, even at elevated temperature, which makes it difficult to properly impregnate a carbon fiber bundle. In addition, the sizing agents of the prior art form a moderately stiff coating and have a very limited stability at the temperature necessary to achieve an acceptable thermoplastic viscosity to facilitate fiber impregnation.
In the prior art, various procedures have been described for the impregnation of fibers by the use of a solution of thermoplastic. However, these procedures are undesirable because of the problems associated with solvent handling such as flammability and toxicity, and because of the need for scrupulous solvent removal from the intermediate product form to avoid deterioration of mechanical properties at elevated temperatures which can result from plasticization or voids.
Accordingly, a need exists for a thermoplastic resin reinforced with carbon fibers which exhibits excellent impact strength, can be formed readily without the need for a curing process, needs no refrigeration, has good solvent resistance and creep resistance, as well as good elevated temperature performance, and which is readily manufactured with a relatively uniform distribution of reinforcing fibers throughout the resin matrix.